Process of preparing steel for enameling



United States Patent PROCESS OF PREPARING STEEL FOR ENAMELING George A. Shepard, Parma, Ohio, assignor to Republic No Drawing. Application September 6, 1957 Serial No. 682,271

3 Claims. (Cl. 204-38) The present invent-ion relates generally to the art of porcelain-enameling ferrous metal articles and is more particularly concerned with novel ferrous metal articles and porcelain-enameled ferrous metal articles and with a unique method of making said articles.

In the enameling industry, it is common practice to provide a ground coat of enamel on a steel article for example, to assure good adherence of the enamel finish coat. This practice, however, requires two separate firing operations and is consequently undesirably slow and expensive. Accordingly, methods have been developed to improve cover coat adherence and enable direct application of cover coats to metal surfaces. But success in this direction has been limited, primarily because these methods effectively preclude a continuous line production operation, there being a number of treatments performed on a given steel article after fabrication of the article is complete. Any continuous line, in other words, would have to be so complex and there would be so many interruptions that it could not be satisfactory in a commercial installation. The single exception to this rule is the novel method disclosed and claimed in application Serial No. 247,466, filed September 20, 1951, by Irving P. Whitehouse et al., now United States Patent No. 2,748,066.

In accordance with this invention, the difficulties which stand in the way of adapting prior art methods to continuous line production have been eliminated and additional advantages have been obtained. Therefore, it is readily possible to establish the process of this invention on a commercial scale in a continuous line and thereby to produce unique and valuable ferrous metal articles.

Generally, the method of this invention comprises the steps of cleaning a metal article, contacting the resulting clean article with an aqueous nitric acid solution, then rinsing the article substantially free from the nitric acid solution, contacting the article with an aqueous sulphuric acid solution, then rinsing the article substantially free from the sulphuric acid solution, contacting the article with an aqueous zinc plating bath and removing the article from that bath when a coating of zinc of substantially uniform critical thickness has been deposited on the article.

The cleaning operation is carried out by any suitable means such as alkaline soak cleaning, electrocleanirig, solvent cleaning, vapor degreasing and the like. In any case, this cleaning is sufl'iciently complete so that the metal surface is substantially free from oil, grease and dirt and displays no water breaks. A water rinse corn-; pletes this step, drying not being necessary when the next operation is promptly begun, as in a continuous processing line.

The complex etching and pickling step is preferably carried out immediately following the cleaning operation described above, the clean article being suitably dipped in an aqueous nitric acid solution of between about 20 percent and about 35 percent strength. The immersion period is between about 5 seconds and about 20 seconds, which assures the creation of surface characteristics which produce good enamel adherence on the article. However, as those skilled in the art will understand, there is some degree of flexibility about this time factor and the nitric acid concentration so that a longer time of immersion will be required for essentially the same etching results in more dilute solution. Conversely, etching will generally proceed more rapidly in stronger solutions and less time will be required for the immersion step.

Upon removal of the article from the nitric acid bath, it is water rinsed free from the adhering nitric acid solution and then promptly immersed in a bath of aqueous sulphuric acid of about 10 percent to 20 percent strength. The period of immersion in the sulphuric acid solution may vary between about 10 and about 20 seconds with satisfactory results, but again, concentration and time may be correlated and varied within certain limits. The object of this step, the art will recognize, is to effect removal of all oxides from the surface of the metal article so as to present a clean metal surface for the subsequent plating step.

Following removal of the article from the sulphuric acid solution, it is water rinsed substantially free from adhering sulphuric acid solution and then promptly immersed in an aqueous zinc plating bath. The zinc is electrolytically applied to the freshly etched surface as a flash coat, being on the order of 10 to 50x 10* inches in thickness. The bath used to carry out this flash coating operation may suitably be of the following composition:

ZnSO .7H O 430 grams per liter. (NH )2SO 30 grams per liter. pH 2-5.

Current densities which are customary in the art in zinc plating may suitably be employed in carrying out this zinc plating operation. Thus the current density will be between about 10 and about 40 amperes per square foot of cathode surface and the voltage will be between about 2 and 5 volts. The temperature of the plating bath may initially be substantially that of the room and it may be permitted to go as high as 100 F. to 125 F. in use. Alternatively, this coating operation can be carried out under these same current, voltage and temperature conditions with a bath of one of the following compositions:

ZnCl 200 grams per liter. NaCl 80 grams per liter. pH 2-5.

ZnCl 200 grams per liter. (NI-10250 30 grams per liter. pH 2-5.

In general, however, any zinc bath suitable for electroplating ferrous articles can be substituted for the above mentioned baths without affecting the efficiency of the zinc coating or the prepared steel surface.

. a zincfiash coat not only is protected as to its prepared surface in storage but also in forming operations, and

that drawing operations and the like are aided by the presence of the zinc coat which has a lubricating eflect in the drawing dies.

When the article is ready for enameling, the zinc flash coat is stripped ofi in any suitable manner, preferably by means of acid pickling and then is presented clean for the deposition on its surface of a coating of nickel of substantially uniform critical thickness between about 0.000002 and about 0.00001 inch. This zinc coating removal is preferably carried out by chemical means and I have found that a sulphuric acid solution of relatively low concentration, as about 5 percent, can be used for rapid stripping the article of the zinc coating, and that this treatment assures the best results in subsequent nickel plating and enameling steps. This invention, however, contemplates removal of the zinc flash coat by other chemical means and by physical means, as those skilled in the art will understand. In any event, the major consideration insofar as this operation is concerned is the restoration of the surface condition which existed on the article just prior to the establishment on it of the zinc flash coat.

The thus prepared article is then treated as promptly as possible, preferably immediately, to provide on the clean surface a coating of nickel or equivalent metal of critical thickness, as stated above. This nickel plating operation is carried out in a suitable bath and is preferably accomplished by electrodeposition or chemical displacement. Alternatively, however, the nickel coating may be provided by chemical reduction in accordance with the procedure described in detail in the Journal of Research, National Bureau of Standards, issues of July 1946 and November 1947. These procedures are set forth in detail in aforesaid Patent No. 2,748,066.

The application of the enamel slip and the firing of the slip on the nickel plated surface of the article is carried out in accordance with the'usual practice in the enameling operations.

The following illustrative, but not limiting, example of this invention as I have actually carried it out or ofiered to explain further to those skilled in the art the nature and advantages of my invention:

Example A cold rolled steel panel was cleaned cathodically in a proprietary alkali cleaner for 30 seconds. It was then rinsed free of all cleaner and immersed in a 30 percent nitric acid solution for seconds. After removal from the nitric acid it was subsequently rinsed and then immersed in a 10 percent sulfuric acid solution at 160 F. for seconds. It was rinsed after this treatment, and then zinc plated to a thickness of 50 millionths of an inch of zinc in a bath of the following composition:

ZnSO .7H O 430 grams per liter. (NH,) 250 30 grams per liter. pH 4.0.

This bath was maintained at room temperature, i.e. about 70 F., during the short period of the plating operation while the current density was amperes per square foot of work piece (cathode) surface at 5.0 volts.

The steel was subjected to the laboratory atmosphere for 30 days; after which a drawing operation was performed on the panel. The zinc plate was then removed with 5 percent sulfuric acid and the piece subsequently rinsed and placed in a Watts nickel bath for 13 seconds at a current density of 30 amperes per square foot. The sample was then rinsed and dried. A titanium acidresistant cover coat enamel was then applied directly over the prepared sample and fired in a manner prescribed by the manufacturer. The resulting porcelain enamel coat was firmly adherent to the base metal and the resulting surface was free of defects.

Throughout this specification and the appended claims, where percentage or proportions are recited, reference is had to the weight basis.

This is a continuation-in-part of my copending application Serial No. 416,705, filed March 16, 1954, andentitled Process of Preparing Steel for Enameling, now abandoned.

Having thus described this invention in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains to make and use the same, and having set forth the best mode contemplated of carrying out this invention, I state that the subject matter which I regard as being my invention is particularly pointed out and distinctly claimed in what is claimed, it being understood that equivalents or modifications of, or substitutions for, parts of the above specifically described embodiments of the invention may be made without departing from the scope of the invention as set forth in what is claimed.

What is claimed is:

1. The method of making a porcelain-enameled, coldrolled steel article which comprises the steps of cleaning the article and removing substantially all oil, grease and dirt therefrom, contacting the resulting clean article for between about 5 and about 20 seconds with an aqueous nitric acid solution of between about 20 percent and about 35 percent strength, thereupon water-rinsing the article substantially free from nitric acid solution, contacting said article for between 10 and about 20 seconds with an aqueous sulphuric acid solution of between about 10 percent and about 20 percent strength and thereby producing a surface on the article which is receptive to the application of an adherent uniform enamel coat, then promptly water-rinsing the said article, contacting said article with an aqueous zinc plating bath and thereby depositiing a coating of zinc on the article, removing the article from the bath when a coating of zinc of substantially uniform thickness of about 0.00001 to about 0.00005 inch has been deposited on said article to preserve the said enamel receptive surface of the article under the zinc coating through a protracted period of time and through shaping and working operations, shaping the article in the finally desired form, chemically stripping substantially all the zinc coating from the article to expose the said enamel-receptive surface, depositing on the resulting exposed enamel-receptive steel surface a nickel coat of substantially uniform thickness between about 0.000002 and about 0.00001 inch, then applyingdirectly to the resulting nickel coating porcelain enamel slip, and firing said slip and producing an enamel coat.

2. The method of preparing a cold-rolled steel article for porcelain enameling which comprises the steps of etching the article to provide surface characteristics conducive to good enamel adherence, removing all oxides from the surface of the article, depositing on the resulting surface of the article a coating of zinc of substantially uniform thickness between about 0.00001 and about 0.00005 inch, and shaping the article into final desired form while the said surface characteristics are preserved under the zinc coating, chemically removing substantially all of the coating of zinc, depositing on said article a nickel coat of substantially uniform thickness between about 0.000002 and about 0.00001 inch, applying a coating of porcelain enamel slip directly to the resulting nickel coating, and firing said slip and producing an enamel coat.

3. The method of preparing a cold-rolled steel article for porcelain enameling which comprises the steps of etching the article to provide surface characteristics conducive to good enamel adherence, removing all oxides from the surface of the article and thereby producing a surface on the article which is receptive to the application of an adherent uniform enamel coat, depositing on the resulting enamel-receptive surface of the article a coating of zinc of substantially uniform thickness between about 0.00001 and about 0.00005 inch to preserve the said enamel receptive surface of the article under References Cited in the file of this patent UNITED STATES PATENTS Madsen Nov. 24, 1925 Conradi et al. July 2, 1940 Sweo Jan. 1, 1952 Whitehouse et al. May 29, 1956 FOREIGN PATENTS Great Britain June 25, 1952 

2. THE METHOD OF PREPARING A COLD-ROLLED STEEL ARTICLE FOR PORCELAIN ENAMELING WHICH COMPRISES THE STEPS OF ETCHING THE ARTICLE TO PROVIDE SURFACE CHARACTERISTICS CONDUCIVE TO GOOD ENAMEL ADHERENCE, REMOVING ALL OXIDES FROM THE SURFACE OF THE ARTICLE, DEPOSITING ON THE RESULTING SURFACE OF THE ARTICLE A COATING OF ZINC OF SUBSTANTIALLY UNIFORM THICKNESS BETWEEN ABOUT 0.00001 AND ABOUT 0.00005 INCH, AND SHAPING THE ARTICLE INTO FINAL DESIRED FORM WHILE THE THE SAID SURFACE CHARACTERISTICS ARE PRESERVED UNDER THE ZINC COATING, CHEMICALLY REMOVING SUBSTANTIALLY ALL OF THE COATING OF ZINC, DPOSITING ON SAID ARTICLE A NICKEL COAT OF SUBSTANTIALLY UNIFORM THICKNESS BETWEEN ABOUT 0.000002 AND ABOUT 0.00001 INCH, APPLYING A COATING OF PORCELAIN ENAMEL SLIP DIRECTLY TO THE RESULTING NICKEL COATING, AND FIRING SAID SLIP AND PRODUCING AN ENAMEL COAT. 